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Decode Ways

Dynamic Programming·Medium

A message containing letters from A-Z can be encoded into numbers using the mapping 'A' -> "1", 'B' -> "2", ..., 'Z' -> "26".

Given a string s containing only digits, return the number of ways to decode it. The answer fits in a 32-bit integer.

Note that any decoding that maps to a leading-zero substring (e.g. "06") is invalid because the original encoding could not produce one.

Examples

Input: s = "12"

Output: 2

"12" decodes as "AB" (1 2) or "L" (12).

Input: s = "226"

Output: 3

"226" decodes as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

Input: s = "06"

Output: 0

"06" cannot decode — leading zeros are invalid.

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Passed32 ms

First attempt — retry to track metric movement here.

Complete approach — pattern and complexity both named.

You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.

No divergence detected. The code correctly aligns with the stated approach.

Try solving related sliding window problems to build familiarity with two-pointer variants.

No signals fired — clean run, you stayed in flow.